The present invention is in the field of semiconductor manufacturing and specifically relates to an apparatus for polishing and uniformly reducing the thickness of a thin film or coating that has been applied to a wafer composed of a semiconductor, typically single-crystal silicon.
Because of their thinness, the wafers are not rigid, but instead are somewhat flexible. However, they are not pliable like paper or metal foil. FIG. 1 shows (not to scale) a typical wafer, including its silicon substrate and the applied coating.
It is important to appreciate the extremely small thickness which the present invention controls. A typical wafer is several inches in diameter and about 680 microns thick; one micron is 1 millionth of a meter. Conventional lapping techniques, such as described in the patents discussed below are capable of producing wafer surfaces that are flat to within 2 or 3 microns. The coating with which the present invention is concerned is typically only about two microns thick. The thickness of the coating is extremely uniform across the wafer; variations in thickness of the coating are on the order of 0.02 micron. The apparatus and method of the present invention permits the thickness of the coating to be reduced to about 0.8 micron while maintaining the uniformity of the thickness of the coating. Further, this degree of control is obtained routinely on a production line using the apparatus of the present invention.
If a coated wafer of the type with which the present invention is concerned is laid on a rigid surface, as shown in the diagram of FIG. 2, and if a downward force is applied at the point indicated, the pressure transmitted to the underlying rigid surface is greatest directly beneath the point where the force is applied. The pressure will diminish at locations laterally displaced from the maximum pressure point.
If the wafer is very stiff, the applied force will be dissipated over a larger area; the pressure acts over a larger area, and because the combined product of pressure times area must equal the applied force, the pressure immediately beneath the point where the force is applied is less than it would be for a wafer that is less stiff.
In contrast, if the wafer of FIG. 2 is less stiff, the applied force will be mainly concentrated near the maximum pressure point, and the pressure there will be much greater than if the material were stiffer. That is exactly what occurs in the thin wafers with which the present invention is concerned.
FIG. 3 shows a wafer (that is assumed to have a planar upper surface) mounted in a conventional lapping apparatus. Here it is clear that material will be removed most rapidly from the high areas A and B, which will render the thin coating less uniform in thickness.
FIG. 4 shows a wafer (that is assumed to have a planar lower surface) mounted in a conventional lapping apparatus. From the discussion in connection with FIG. 2 it is clear that even though the lower surface of the wafer is assumed to be planar, material will be removed at different rates at various lateral positions, thereby rendering the thin coating less uniform rather than more uniform.
Reflecting upon the above discussion in connection with FIGS. 1-4, it is seen that so long as the thickness of the wafer is nonuniform, the use of a rigid carrier to force the wafer against the polishing surface necessarily results in nonuniform removal of the material, which destroys the uniformity of the thickness of the coating.
To remedy this situation, it is a practice in the art to include an insert consisting of a sheet of a resilient material between the carrier and the wafer. This tends to spread the applied downward force a little more uniformly across the wafer. The use of such an insert, such as in U.S. Pat. No. 5,205,082 discussed below, can never be fully effective because the insert is compressed most at the high spots and therefore the elastic restoring forces are also greatest at the high spots. Since the applied force is also greatest at the high spots, the use of an insert is only marginally effective. Further, the use of an insert complicates the mass-production process, since the inserts need to be changed from time to time and because they must be carefully installed.
The arrangement shown in FIG. 3 is, of course, the conventional way of producing flat surfaces, and by extension, parallel surfaces. However, as stated at the outset, neither of these is the purpose of the present invention. Instead, the purpose of the present invention is to uniformly reduce the thickness of a thin coating previously applied to one face of a wafer.
For this reason it is not surprising that the known prior art, which was concerned with producing flat surfaces or parallel surfaces, is not useful in solving the problem with which the present invention is concerned.